As already mentioned, the observations of the Magellanic Clouds with
the Parkes 210-foot dish have the highest physical resolution so far
attained. The Magellanic Clouds cover some 800 square degrees of sky
and, at a distance of
50 kpc, are the
nearest extragalactic objects to our own galaxy. A low-resolution
(2.°2 beamwidth) HI survey
(Hindman, Kerr, and McGee,
1963)
showed that the Large and Small Clouds (LMC and SMC) are embedded in the
same HI envelope. The total HI mass of the system is some
1.5 × 109M;
5.4 × 108M is
associated with the LMC and 4.8 × 108M with
the SMC.

The 210-foot survey of the LMC
(McGee and Milton, 1966)
revealed 52 HI complexes of mean HI mass 4 × 106M and
diameter 575 pc ( 1 HI
atom/cc). The HI complexes
are closely associated (in position and velocity) with HII regions and
OB stars, but do not correlate with the stellar clusters.

Of 90 large HII regions cataloged by
Henize (1956),
61 are closely associated with the HI complexes. The supergiant OB stars
with measured velocities also have a high
correlation in position and velocity with the HI complexes.

From an analysis of the rotation of the
LMC and the position and velocity distribution of the population I objects,
McGee and Milton (1966)
proposed a spiral structure for the LMC. The total mass derived from
rotational and random motions is greater than 6 × 109M,
giving the LMC a fractional hydrogen content
of 5 to 9%. The 1410-MHz radio continuum also correlates well with the
integrated hydrogen contours.

The neutral hydrogen distribution in the SMC is rather smooth, with
three major concentrations reaching peak brightness temperatures of 150,
110, and 100°K, merging into a high-level background. This smooth
distribution is in striking contrast to the clumpy distribution in the
LMC. The most interesting feature of the 210-foot
survey
(Hindman, 1967)
is the presence of what would appear to be three massive (1 to
2 × 107M)
expanding neutral hydrogen shells of diameter 1 to 2
kpc, which could be the result of supernova explosions. The SMC is also
thought to be rotating, but is only slightly flattened. The estimated
mass is 1.5 × 109M, with
some 30% neutral hydrogen. As
in the LMC, there is a correlation between the distribution
of neutral
hydrogen and the bright stars. About half of the cataloged HII regions
(Henize, 1956)
are located in the vicinity of the three major gas concentrations.